Abstract:

Over the last few decades medical ultrasound has become an established diagnostic and therapeutic tool since it produces detailed and high resolution images of tissues in human body. Harmonic imaging is among the recent developments which has brought further improvements to the image quality. Harmonic imaging for tissue or with contrast agent induced a rapid evolution of this modality to diverse clinical uses, among which myocardial perfusion determination seems to be the most important application. This brought the need to understand the physical processes involved in the propagation of finite amplitude sound beams, and the issues for redesigning and optimizing the transducers with higher performances for both tissue imaging and contrast imaging. Concerning tissue harmonic imaging, the advantage of the harmonic beam generated at two times the transmit frequency are translated by reduced reverberations , greater depth of penetration at higher frequencies and improved resolution. In order to characterize the harmonic beam, a time domain solution of the parabolic nonlinear wave equation is used. This equation is traditionally applied in a propagation direction along the central transducer axis, and has been shown to model the pulse propagation satisfactorily. In this work, the characteristics and performances of the second harmonic acoustic beam from a focused piston aperture are described and the physical principles behind tissue harmonic imaging are computed. The field properties are then discussed regarding image quality. Special attention is given to the transmitted and received bandwidths variation and the reception of the pure echo signal.

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